1. Field of the Invention
This invention relates to the recovery of bitumen from rocks or sands and more particularly to a process and apparatus which results in increased recovery of bitumen from such sand using direct thermal separation. It also relates to the thermal cracking of hydrocarbons and recovery of the cracked products.
Large deposits of bituminous sand and shales are found in various localities throughout the world. The term "tar sand and ore" is used herein to include those materials containing hydrocarbons in various host materials. One of the most extensive deposits of tar sand occurs, for instance, in the Athabasca district of the Province of Alberta, Canada.
Typically the composition of these sands contain, by weight: from about 5% to about 20% of oil; from about 1% to about 10% of water; and from about 70% to about 90% of inorganic solids. The specific gravity of the bitumen varies from about 1.0 to about 1.05. The specific gravity of the bitumen as well as all other values of specific gravity given herein are taken at 60.degree. F. The major portion, by weight, of the inorganic solids is fine grain quartz sand having a particle size greater than about 45 microns and less than 2000 microns. The remaining inorganic solid matter has a particle size of less than 45 microns and is referred to as fines. The fines contain clay and silt including some very small particles of said. The fines content typically varies from about 10% to about 30% by weight of the solid inorganic content of bituminous sand. The true specific gravity of the sand is about 2.65 whereas that of the fines is about 2.7. This typical composition is only exemplary and it will be appreciated that the composition of bituminous sand varies from the stated ranges. For example, in mining the bituminous sand, clay which is found in layers of varying thickness in such sand areas may be admixed with the bitumen, thus increasing the inorganic solids content and particularly the fines content of the material to be processed.
In one aspect of the invention, part of the apparatus described, in modified form, is used to treat heavy hydrocarbon - containing feed stock. In this treatment, at least part of the hydrocarbons are cracked and the gaseous products are recovered. The feed stocks which have been used for this purpose include the wellhead products obtained by in situ recovery of heavy oil and refinery tower bottoms. Typically the hydrocarbon in such feed stocks may in large part be characterized by an API gravity less than about 25.degree. API.
2. Brief Description of the Prior Art
Various methods have been proposed for separating bitumen from bituminous sand. The two best known methods are often referred to as the "hot water method" and the "cold water method." In the former, the bituminous sand is jetted with steam or hot water and mulled with a small proportion of water at about 175.degree. F. The pulp is then dropped into a turbulent stream of circulating water and carried through a separation cell maintained at an elevated temperature of about 180.degree. F. In the separation cell, entrained air causes the oil to rise to the top in the form of a froth rich in bitumen which is then drawn off.
The so-called "cold water method" does not involve heating the bituminous sand other than whatever heating might be required to conduct the operation at room temperature. The process involves mixing the bituminous sand with water, soda ash and an organic solvent such as kerosene. The mixture is then permitted to settle at room temperature. A mixture of water and bitumen dissolved in the organic solvent rises to the top of the settling zone and is recovered.
Both the hot water and cold water processes mentioned, as well as many variations on these processes and other processes suggested for recovery of bitumen from bituminous sand, involve a settling or separation step in which a fluid slurry of bituminous sand is introduced into a body of water so that bitumen rises to the top of the body of water, as a froth or a diluted oil, which sand settles to the bottom. In all such processes, the separation of oil from solids is not complete and some bitumen and solids, mostly fines, remain in the water. Likewise, the bituminous froth and ultimately the bitumen recovered from the process contains solids. Separation of both water and mineral solids from the froth is necessary for most ultimate uses of the bitumen, such as upgrading in conventional refining operations. Attempts to reduce the solids content of recovered bitumen usually result in an increase in the amount of bitumen associated with fines in the water discarded from the process, thus reducing bitumen recovery and polluting the water.
The described processes also produce a tailings sand that must be contained in tailings dams and result in very large water settlement areas. They are also relatively poor in percentage recovery of bitumen, and have major power and heat requirements for conducting the process.
U.S. Pat. No. 3,481,720 to Bennett describes a process and apparatus for removing hydrocarbons, by direct thermal conversion, from oil shale, oil sands and the like. The patent describes an apparatus in which several concentric cylindrical structures are made to rotate about their common, horizontal cylindrical axis. The feed material enters one end of the central cylinder and is moved through the cylinder as it rotates. An auger-like structure which extends from wall to wall within the cylinder provides the advancing means for moving the feed through the cylinder. Feed material is progressively heated as it advances through the inner cylinder and steam generated by this preheating is extracted by a small conduit extending axially through the cylinder. The preheated feed material advances into a retort zone provided by a larger cylinder surrounding the inner cylinder. In this area, hydrocarbon vapors are produced and withdrawn from the apparatus. A second auger-like drive, which also extends from wall to wall in the retort zone, advances the feed material through the cylinder toward the product removal end of the apparatus where the inorganic host material and its carbon residue is dropped into a larger surrounding cylinder which defines a combustion zone. A similar wall-to-wall auger drive arrangement in the combustion zone causes the material to reverse its direction of travel. The carbon content of the inorganic residue material is burned to provide heat for the process and the resulting carbon free heated material is partially recycled to the retorting zone. The unrecycled portion of the material is ejected from the feed end of the apparatus for disposal. Various heat exchange means are described for salvaging the heat content of the exhaust gases and solid waste emitted from the apparatus.
The Bennett apparatus relies on the presence of the feed material within the auger arrangement to provide a physical seal which prevents hydrocarbon vapor from flowing into the combustion zone and prevents oxygen in the combustion zone from entering the area where the hydrocarbon vapors are being produced. The feed material must of necessity be such as will advance through the restricted flow passages formed by the various auger drives.
While the proposed Bennett process eliminates certain of the problems and limitations present in the hot water and cold water methods described previously, the patented apparatus and related process include various limitations and inherent problems, especially when used with sticky, lumpy materials such as tar sands. An initial limitation is that the feed material must be initially processed so that the feed particles are small enough to move through the limited helical passageways provided between the cylinders and the auger or bladetype advancing elements described in the patent. The design of the Bennett structure is also such that little, if any, grinding or agitation is imparted to the particles as they advance through the apparatus so that any size reduction of the feed is limited to the effects of heating and mixing with steam or other gases.
Where tar sands or similar sticky feed materials are to be processed, the large surface areas associated with the auger and paddle advancing mechanism of the Bennett apparatus increase the possibility of sticking and plugging. There is no control on the recirculation of fines into the vapor production zone which increases the likelihood of contaminating the hydrocarbon vapors. In general, while the Bennett process and apparatus does not suffer from certain of the problems inherent in prior art attempts at removing hydrocarbons from tar sands and other ores, the patented process and apparatus nevertheless have various practical limitations which could render them undesirable or unsuitable for processing such ores.